1. Field of the Invention
The present invention relates to the optical recording and/or playback of high density information and more particularly to improved methods, media and apparatus for implementing such recording and/or playback in a flexible disc format.
2. Description of the Prior Art
The currently preferred optical disc technology employs disc elements with spiral or concentric tracks of minute (usually on the order of a micron or less in size), optically-detactable marks. One realtime mode of recording (writing) such marks is by scanning tracks on the disc with an equivalently small beam of radiation (e.g., from a laser) which is modulated "off or on" according to an electrical signal representative of the information to be written. Information is recovered (read) by scanning the tracks with a larger, but still very tightly focused light spot. The recovered information is in the form of a fluctuating electrical signal obtained from a photodetector that senses the read-out light reflected from the recorded disc.
In order to write and read information in the form of such minute markings, optical systems of high numerical aperture are used to focus light to equivalently minute spots. Such optical systems have extremely small depths of focus and the proper positional relation between the writing or reading optical system and the optical disc surface must be stringently maintained. One approach has been to utilize highly-precise turntables for supporting and rotating the optical discs in a predetermined plane. Such apparatus is expensive, but even with such costly turntables it is usually necessary also to provide complex focus-servo devices which effect lens adjustment in response to minute variations in the position of the surface of the optical disc relative to the lens. Such variations are caused by thickness variations and non-flatness of the disc or turntable surface or apparatus vibrations. Since the discs rotate at high speeds (e.g., 1800 RPM), these focus-servo devices must respond at very high frequencies (e.g., 500 Hz. for discs with ground glass substrates and in the range of 1000 to 3000 Hz. for discs with molded plastic substrates). Therefore these focus-servo devices are also costly, and fragile.
The costs and care of such sophisticated writing and reading apparatus have not presented insurmountable problems for some industrial or laboratory applications, but do present significant obstacles for consumer and business systems applications. Therefore, an alternative has been suggested whereby master discs, formed with the sophisticated apparatus described above, will be used to form replicate discs on flexible substrates, such replicate discs being intended for playback only. It is suggested that such flexible discs be air-supported during rotation in a manner similar to that previously used for magnetic discs. That is, it had been known previously that a "floppy" disc, comprising a magnetic layer on a readily-flexible, plastic support, could be rotated in a generally stable condition by providing an opposing stationary reference surface and supplying a throttled air flow between the disc and reference surface from a location near the rotational center to the disc periphery (see, e.g., U.S. Pat. No. 3,208,056).
The approach described in the above noted patent relied on the rotating disc achieving a balance of dynamic, fluid, gravitational and elastic forces so as to generally stabilize in some equilibrium range when rotated at a fixed speed. In accordance with the teachings of that patent, fluid forces were regulated (by controlling the air flow between the disc and stabilizing reference surface) to selectively position the disc relative to magnetic transducers embedded in the stabilizing reference surface. This allowed compensation for the differences in different discs' equilibrium positions or changes in ambient fluid conditions. It is significant to note that the aim for this magnetic write-read application was to provide a disc to transducer spacing of 0.001" or less and that the stabilizing reference surface included cavities as well as the embedded transducer heads. In a subsequent patent (U.S. Pat. No. 3,191,179) of the same assignee it was pointed out that there still existed instability problems in devices of the kind disclosed in U.S. Pat. No. 3,208,056. It was suggested that electrostatic forces be used to remedy such problems.
Subsequent development of this aerodynamic stabilization approach (see U.S. Pat. No. 3,178,719 of the same assignee) led to the implementation of what was termed a "regulated mode" of operation. In accordance with the regulated mode, the rotating disc was generally stabilized as described in U.S. Pat. No. 3,208,056 and then locally regulated or constrained toward the magnetic heads, e.g., by a localized vacuum proximate the head.
In prior art attempts to adapt the magnetic disc aerodynamic stabilization for the reading of optical discs, the regulated mode approach has been pursued. More specifically, although there has been disclosed various embodiments of optical disc reading apparatus which utilize a stationary reference, a rotating flexible disc and an air flow therebetween, all such embodiments additionally utilize some localized regulating means at the optical detection site to constrain the disc into a precise focused position relative to the optical system. Typical examples can be seen in French Pat. No. 2,167,258 (wherein a dihedral-shaped reference surface and localized stabilization plates effect such constraint) and in U.S. Pat. No. 3,947,888 (wherein a negative pressure differential in the vicinity of the reading head constrains the disc into a predetermined head-to-disc spacing). An interesting commentary on the problems which have existed is in "The Aerodynamic Stabilization of Video Discs", IEEE Transactions on Consumer Electronics, Vol. CL-21, No. 2, May 1975, wherein another "regulated mode" embodiment combining aerodynamic prestabilization and localized constraint is disclosed.
Thus, prior art techniques for rotating flexible optical discs on an air cushion have followed the regulated mode approach with various localized disc-constraining devices. Such devices add to the cost and complexity of the apparatus. Moreover, even with such constraining devices, prior art techniques have only been suggested for use in reading (not writing) with flexible optical discs. The focus precision requirements for writing on optical discs are even more stringent than for reading such discs. For example, typical optical disc systems for reading would have depth of focus tolerances of from .+-.3.mu. to .+-.7.mu. (depending on the information storage density of the disc), whereas the corresponding systems for writing information on such discs would have tolerances of from .+-.0.25.mu. to .+-.1.mu..